Whereas some would consider symbols a doorway to an archetypal space, wherein we perform the necessary alchemy (psychological processing) at a higher rate than mere reality would permit, this movie suggests that its symbols encrypt a literal truth, and so points us back to another ploddingly special case reality.
The mundane cop show reality in this case: the existence of a secret service, aka the Priory of Scion and/or Knights Templar, charged with protecting Jesus of Nazareth's literal descendents. The grail is not a literal cup silly, but Jesus's wife Mary Magdalen, her corpse concealed somewhere, and presumably still a source of testable DNA.
For centuries, talk of "blood lines" occluded the actual genetic science involved, but now, in time for the millenium, we might follow clues to finally solve this very cold case, thereby undermining the standard version of the story, in which Jesus was childless. Opus Dei doesn't want this case solved, and sends out a mentally troubled assassin to keep the myth of "childless Jesus" intact. Tom Hanks, a specialist in religious symbols, is unwittingly caught in the middle of this game.
Theologically speaking, it's unclear why Jesus having children would undermine the Trinitarian model, as his mortal shell was never considered dysfunctional in any way. He just never had the time for fatherhood. In a more civilized part of the world, he might have, but the outskirts of Imperial Rome was very unstable, very unsafe for anyone practicing high level leadership, especially if Jewish.
In other words, there's no scientific evidence that his divinity was genetically based, so the fact of French descendents sharing some DNA would not thereby communicate his special status as a savior. There'd need to be more in the way of skills and core teachings to keep his legacy alive, but shared genes would likely have nothing to do with it, so why should the Church feel so threatened by his literal offspring? Let's just update the story and get on with it.
I think it's a safe bet that if we could trace his descendents by means of DNA, they'd be celebs, given the way geneology buffs like to point to famous ancestors in any given tree. But would we really need a secret service to protect their identities? Why? Is Imperial Rome still that much of a concern?
Of course one might take this whole second literal truth of the Holy Grail (not a cup for wine-into-blood, but a container for genetic material) and repoint it back to the nonliteral archetypal realm. In that case, it's more a story about the central importance of women in the Christian tradition, right back to Jesus himself, who loved his women friends as much as he loved anyone. Is that undermining? I suppose to some.
The movie encourages this symbolic interpretation by framing a lot of art, implying the movie screen is itself a kind of canvas on steroids, a source of psychological and/or theological insights. One could make the same moves to rescue National Treasure, another puzzle-solving challenge leading to a fictitiously literal outcome.